Digital systems rely on precise clocks for sequencing among operating states. Higher-speed systems have required faster and faster clock rates. Crystal oscillators are often used to generate these clocks. A piezoelectric effect causes a crystal such as quartz to vibrate and resonate at a particular frequency. The quartz crystal naturally oscillates at a particular frequency, its fundamental frequency that can be hundreds of megahertz.
The frequency of oscillation can be adjusted somewhat by adjusting the voltage bias to a varactor on the crystal's terminals. However, frequency adjustment is much less than 1 percent. Such voltage-controlled crystal oscillators (VCXO) are popular for their ease of output-frequency adjustment.
Very high-speed systems may require clocks that are faster than the fundamental clock rates of commonly-available crystal oscillators. Sometimes the clock output can be multiplied, such as by using a phase-locked loop (PLL). However, the feedback within the PLL can limit performance or cause distortions or other effects and costs can increase.
The fundamental frequency of oscillation of the crystal may be limited by various factors, such as the geometry of the crystal. Higher frequencies may require thinner crystals that are much more expensive to manufacture. Thus crystals are currently limited to frequencies of less than 200 MHz for inexpensive crystals, or 500 MHz for expensive crystals.
Prior-art ceramic coaxial resonators achieve higher-frequency operation essentially by physically dividing the resonator in half. Since the frequency of resonance is set by the coaxial length, the output frequency is doubled with the halving of the resonance element. In contrast, crystal oscillators are fabricated from synthetic quartz material. Such a Bulk Acoustic Wave (BAW) crystal oscillator has a resonating frequency determined by its thickness. Unlike coaxial resonators, BAW crystal oscillators have higher resonating frequency modes, or overtones, that are inherent within the BAW element. Such overtones are strictly limited to odd integer multiples.
For optimum frequency stability and low phase noise, BAW crystal oscillators having a high quality (Q) factor are more desirable that coaxial resonators with lower Q factors. However, BAW crystal oscillators typically operate at much lower frequencies than coaxial resonators.
What is desired is a crystal oscillator that outputs a faster clock than the fundamental frequency of the crystal. A crystal oscillator circuit that operates at odd harmonic multiples of the fundamental crystal frequency is desirable. A crystal oscillator that operates at the third overtone and then has its output doubled to produce a 6× output is desired.